Field of the Invention
The present invention relates to self-excited flyback switching power supply apparatuses.
Description of the Related Art
A self-excited flyback switching power supply has been known from the past as a low-voltage power supply for an electronic apparatus. FIG. 5 is a basic circuit diagram of a self-excited flyback switching power supply. Referring to FIG. 5, the switching power supply includes a commercial alternating current (AC) source 400, a filter circuit 401, a rectifier circuit 402, and a primary electrolytic capacitor 403. The AC voltage input from the commercial AC source 400 passes through the filter circuit 401 and is converted to direct current (DC) voltage by the rectifier circuit 402 and smoothing capacitor 403. The power supply further includes a transformer 419, a primary winding Np of the transformer 419, a starting resistance 406, an auxiliary winding Nb wound around the primary side of the transformer 419, a first switching element 405, and a resistance 407 provided between a gate and a source of the first switching element 405. The primary winding Np of the transformer 419 and the switching element 405 are connected in series. The starting resistance 406 is connected to between a positive terminal of the capacitor 403 and a gate terminal of the switching element 405. When the gate voltage of the switching element 405 gets higher than the DC voltage of the capacitor 403 through the starting resistance 406, drain current flows, and the current is fed to the primary winding Np. As a result, the transformer 419 is excited, and voltage is induced to the other primary or auxiliary winding Nb. The gate voltage of the switching element 405 increases, and the switching element 405 has an ON state.
On the other hand, the auxiliary winding Nb is also fed to a time constant circuit including the resistance 416 and capacitor 415 and is connected such that the voltage across the capacitor 415 may also be applied to between the base and emitter of the transistor 409.
When the voltage of the capacitor 415 increases and the transistor 409 is turned on, the gate voltage of the switching element 405 decreases, and the switching element 405 is turned off.
When the switching element 405 is turned off, the terminal voltage of a secondary winding Ns on the secondary side of the transformer 419 inverts, and current flows out from the secondary winding Ns through a secondary rectifier diode 417. The current is charged to a capacitor 418. The energy stored in the transformer 419 is charged to the capacitor 418 under the limitation with the inductance of the secondary winding Ns. The drain voltage of the switching element 405 while the switching element 405 is having an OFF state is equal to the sum value of the voltage resulting from the multiplication of the voltage of the secondary side by the ratio of the number of turns of the primary winding Np and the number of turns of the secondary winding Ns and the voltage charged in the capacitor 403. When the current of the secondary winding Ns is equal to 0, the voltage generated on the drain side of the switching element 405 starts vibrating about the voltage charged to the capacitor 403 for a period depending on the inductance of the transformer 419 and the capacitor 404.
The voltage of the primary winding Np is reflected on the auxiliary winding Nb. When the drain terminal voltage gets lower than the voltage across the capacitor 403, voltage is applied to the auxiliary winding Nb such that the gate terminal voltage of the switching element 405 may be higher than the source terminal. When the gate terminal voltage exceeds the gate threshold voltage of the switching element 405, the switching element 405 is turned on again. After this point, the operations as described above are repeated.
When the voltage across the capacitor 418 increases, the partial pressure of the resistances 421 and 422 operates a shunt regulator 420, and current is fed to a photo-coupler PC 401. The photo-coupler PC 401 lights up, and the impedance of the phototransistor of the photo-coupler PC 401 decreases. As a result, the voltage of the capacitor 415 of the time constant circuit increases earlier than that charged by the resistance 416, and the transistor 409 is turned on. Thus, the switching element 405 is turned off. This feedback operation allows the switching power supply to output a constant voltage.
Recently, the reduction of power consumption while various electronic apparatuses have standby states has been demanded. An electronic apparatus having the aforementioned self-excited flyback switching power supply has a mode for normal operations (hereinafter, also called a normal mode) and also a power saving mode for standby states (also called a power saving mode). In the power saving mode, the output voltage of the power supply is reduced, and the power consumption at the standby states is reduced.
FIG. 6 illustrates a circuit diagram of a switching power supply in the past. FIG. 7 illustrates waveforms when the output voltage is reduced in the power saving mode in a self-excited flyback power supply. In addition to the self-excited flyback power supply in FIG. 5, the switching power supply in FIG. 6 further includes an output variable circuit having a resistance 421 (resistance value Ra), a resistance 422 (resistance value Rb), a resistance 423 (resistance value Rc), a resistance 424, and a switching element 425. The output variable circuit receives from a central processing unit (CPU) 1, which is a control unit of the electronic apparatus, a power save signal (hereinafter, called a /PSAVE signal) which instructs the shift to the power saving mode. The CPU 1 uses the /PSAVE signal to shift the electronic apparatus from the mode for normal operations to the power saving mode. In order to set the electronic apparatus to the normal mode, the CPU 1 changes the /PSAVE signal to a High level (hereinafter, called an H level). In order to set it to the power saving mode, the CPU 1 changes the /PSAVE signal to a Low level (hereinafter, called an L level). The /PSAVE signal is supplied to the switching element 425. In the normal mode, that is, when the /PSAVE signal has the H level, the switching element 425 is turned on, and the resistance 422 (Rb) and resistance 423 (Rc) are connected in parallel. The voltage resulting from the division of the output voltage by the parallel resistance (Rb//Rc) of the resistance 421 (Ra), resistance 422 and resistance 423 is supplied to the ref terminal of the shunt regulator 420. When the reference voltage of the shunt regulator is Vref, the output voltage Vout-h in the normal mode is substantially expressed by the following expression.
                              V                      out            -            h                          ≅                                                            R                a                            +                              (                                                      R                    b                                    //                                      R                    c                                                  )                                                    (                                                R                  b                                //                                  R                  c                                            )                                ·                      V            ref                                              (        1        )            
In this case, (Rb//Rc) is a parallel resistance value of Rb and Rc and may be expressed by the following expression.
                                          R            b                    //                      R            c                          =                                            R              b                        ·                          R              c                                                          R              b                        +                          R              c                                                          (        2        )            
On the other hand, in the power saving mode, that is, when the /PSAVE signal has the L level, the switching element 425 is turned off, and the resistance 423 (Rc) is isolated. Thus, the voltage resulting from the division of the output voltage by the resistance 421 (Ra) and resistance 422 (Rb) is supplied to the ref terminal of the shunt regulator 420. The output voltage Vout-l in the power saving mode may substantially be expressed by the following expression.
                              V                      out            -            l                          ≅                                                            R                a                            +                              R                b                                                    R              b                                ·                      V            ref                                              (        3        )            
This expression describes that the output voltage Vout-l in the power saving mode is lower than the output voltage Vout-h in the normal mode. When the switching element 405 has the off state, the voltage Vnnl induced in the auxiliary winding Nb is reduced, as substantially expressed by the following expression.
                              V          nnl                ≅                              (                                          V                                  out                  -                  l                                            +                              V                f                                      )                    ·                                    N              b                                      N              s                                                          (        4        )            
As described above, in the power saving mode, the output voltage Vnnl decreases and keeps a relatively low voltage value and has a small amplitude. Thus, the gate voltage of the switching element 405 is lower than the threshold value. This may prevent the switching element 405 from being turned on by the flyback voltage Vnnh. As the path for increasing the gate voltage of the switching element 405 in the power saving mode, the gate voltage is increased through the starting resistance, and the switching element 405 is turned on. When the turning on of the switching element 405 delays and the OFF period of the switching element 405 extends, the oscillating frequency decreases. In this way, reducing the oscillating frequency and reducing the output voltage may improve the circuit efficiency and may reduce the power consumption at the standby states. Japanese Patent Laid-Open No. 2000-278946 discloses the operations at a standby state.
Attempting to reduce the output voltage for power saving in the power saving mode in the configuration in FIG. 6 may limit the voltage reduction as will be described below.
In FIG. 6, the starting resistance 406 has a resistance value R1, and the capacitor 410 has a capacitance C. DC voltage V1 is charged to the smoothing capacitor 403 and is generated across it. In a self-excited flyback power supply which reduces the output voltage in the power saving mode, the output voltage Vout-l decreases, and the drain-source voltage generated when the switching element 405 has the OFF state decreases in the same manner as in the example in the past. This reduces the voltage Vnnl induced in the auxiliary winding Nb of the transformer, and the amplitude of the voltage in the ringing period t2-t3 of the gate-source voltage Vgs of the switching element 405 becomes equal to or lower than the threshold voltage of the switching element 405. Therefore, the only path for turning on the switching element 405 is the increase of the gate voltage through the starting resistance of the resistance 406.
The gradient (in the period t4-t5) of the increase of the gate voltage of the switching element 405 depends on the starting resistance 406 (resistance value R1) and the capacitance C of the capacitor 410 and may be substantially expressed by the following expression.
                              V          gs                ≅                  ·                                    V              1                                                      R                1                            ·              C                                ·          T                                    (        5        )            
In order to reduce the power at the standby states in the power saving mode, the starting resistance 406 operates to increase the operational efficiency of the circuit. This causes the gate voltage of the switching element 405 to increase with a mild gradient. The gate voltage of the switching element 405 increases through the starting resistance 406, and the drain current flows from the primary winding Np. Voltage is induced in the auxiliary winding Nb, and the gate voltage is increased. During this (period t4-t5), the switching loss increases. In the self-excited flyback power supply, the switching loss (increase of the time for turning on) for turning on the switching element 405 in the power saving mode may limit the amount of power reduction at standby states. In other words, further power reduction becomes difficult in the power saving mode.